14 ENGINEERING ON THE FARM 



ioo feet in length. For land-survey work it is 66 feet long. 

 A steel tape is an accurate instrument for measurement, but 

 is rather delicate and requires careful handling. It rusts 

 when exposed to moisture and is easily broken when driven 

 over, tramped upon, or pulled when it has crossed itself. 

 The principal error in the use of the steel tape is due to 

 changes in temperature. Tapes are accurate with a 1 2 -pound 

 pull when the temperature is 62 Fahrenheit. A 10-degree 

 change in temperature above or below 62 ° will lengthen or 

 shorten a 100-foot tape 0.0756 inch. This is not appreciable 

 in a single tape length, but when a measurement is carried 

 through for a mile the accumulated error is 4 inches. It 

 can readily be seen that if a line is measured when the tem- 

 perature is 90 and then remeasured when the temperature 

 is io° below zero, there Will be considerable difference in the 

 two lengths owing to expansion and contraction, although 

 the measurements are made by the same man with the same 

 tape. It is difficult to stretch a tape in a high wind, and in 

 measurements made through tall grass, weeds, or bushes 

 considerable time is required to keep the tape straight, as 

 it has a tendency to lodge in vegetation, and pulling from the 

 ends does not straighten it. 



Chains. The surveyors' and the engineers' chains are 

 made of either iron or steel, the longer parts of the links 

 being connected by three small rings known as joints. This 

 number of loose parts in each joint necessary to make the 

 chain flexible introduces eight wearing surfaces. When each 

 of these surfaces is worn one-hundredth part of an inch, the 

 resulting error in a 100-foot chain is 8 inches. Grass and 

 mud collecting in the joints of a chain may cause a short 

 measurement by preventing the wearing surfaces from com- 

 ing into contact. This is frequently the cause of error when 

 there is moisture on the vegetation or on fresh earth. Pull- 

 ing the chain around a fence post or small tree may shorten 

 the entire chain by bending the links. The pull in stretch- 

 ing the chain is not sufficient to straighten bent links. In 



